In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image consisting of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing only ink is supplied to the master.
The so-called “analogue” printing plates are generally obtained by first applying a so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called “computer-to-plate” (CtP) method has gained a lot of interest. This method, also called “direct-to-plate”, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided in three categories; (i) silver plates, working according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates containing a photopolymerizable composition that hardens upon exposure to light and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion.
Photopolymer plate precursors can be sensitized for blue, green or red light (i.e. wavelength range between 450 and 750 nm), for violet light (i.e. wavelength range between 300 and 450 nm) or for infrared light (i.e. wavelength range between 750 and 1500 nm). Lasers have become the predominant light source used to expose photopolymer printing plate precursors. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate precursor. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been realized using an InGaN material. For this reason, photopolymer plates having their maximal sensitivity in the 350 nm to 450 nm region have been developed during the last years. An advantage of violet photopolymer technology is the reliability of the diode laser source and the possibility of handling the non-developed photopolymer plate precursors in yellow safelight conditions. The use of infrared lasers also became more important in the last years, for example the Nd-YAG laser emitting around 1060 nm but especially the infrared laser diode emitting around 830 nm. For these laser sources, infrared sensitive photopolymer plate precursors have been developed. The major advantage of infrared photopolymer technology is the increased possibility to handle the non-developed photopolymer plate precursors in daylight conditions.
After image-wise exposure of a photopolymer plate precursor a rather complex processing is typically carried out. A pre-heat step is usually carried out to enhance the polymerization and/or crosslinking in the imaged areas. Then, during a pre-wash step, typically with plain water, the protective layer of the photopolymer plate precursor is removed. After the pre-wash step the non-imaged parts are removed in a development step, typically with an alkaline developer having a pH >10. After the development step, a rinse step, typically with plain water, and a gumming step is carried out. Gumming protects the printing plate during the time between development and printing against contamination, fingerprints, fats, oil or dust, or against damage, for example during handling of the plate. Such processing of photopolymer plates is usually carried out in automatic processors having a pre-heat section, a pre-wash section, a development section, a rinse and gum section and a drying section.
To avoid this complex, time consuming and environmentally unfriendly processing of photopolymer plate precursors several alternatives have been described.
In U.S. Pat. Nos. 6,027,857, 6,171,735, 6,420,089, 6,071,675, 6,245,481, 6,387,595, 6,482,571, 6,576,401 and 6,548,222 a method is disclosed for preparing a lithographic printing plate wherein a photopolymer plate precursor, after image-wise exposure, is mounted on a press and processed on-press by applying ink and fountain to remove the unexposed areas from the support. Also US2003/16577 and US2004/13968 disclose a method wherein a plate precursor comprising a photopolymerizable layer can be processed on-press with fountain and ink or with a non-alkaline aqueous developer.
In WO2005/111727 a method is disclosed wherein a photopolymer plate precursor is developed by applying a gum solution to the precursor.
The gum solution, for example a gum solution used in the gumming step of a conventional processing method, is used for both developing, i.e. removing the non-imaged parts of the coating, and gumming the exposed photopolymer plate precursors. According to this method, no pre-wash step, no rinse step and no additional gum step is needed anymore during processing. This method thus provides a simplified processing of photopolymer plate precursors and in addition, since on the one hand no highly alkaline developer is used anymore and on the other hand much less processing liquids are used altogether (no pre-wash, no rinse and no separate gumming), provides an environmentally more friendly processing. WO2007/057334 also discloses a method to prepare photopolymer plates wherein the development is carried out with a gum solution. However, in this method a pre-wash is carried out before development with the gum solution. Other methods, all using a gum solution to develop photopolymer plate precursors, are disclosed in for example WO2007/057335 and WO2007/057349. WO2007/057348 and WO2007/057336 disclose a method wherein a gum solution is used to develop a photopolymer plate precursor and wherein a pre-heat step is carried out after exposure and before development. In WO2007/057336, the pre-heat section and the development section are combined in one single apparatus. Development with the gum solution in the above mentioned methods is usually carried out at room temperature.
Typically, in the above described methods, a pre-heat step is carried out between exposure and development. Providing a pre-heat step after exposure and before processing of a photopolymer printing plate precursor is well known in the art. The pre-heat treatment accelerates the polymerization and/or cross-linking in the imaged parts of the precursor, thereby increasing the durability and improving the hardness of the imaged parts. This may result in an increased run length, i.e. number of high quality prints that can be obtained with a single printing plate.
During such a pre-heat step, the plate is typically kept at a plate surface temperature, measured on the back side of the plate, ranging from 70° C. to 150° C. for a period of one second to 5 minutes using heating means such as a conventional convection oven, IR lamps, UV lamps, an IR laser, IR tiles, a microwave apparatus or heated rollers, for example metal rollers.
To further simplify the method of preparing photopolymer printing plates it would be advantageous if the pre-heat step could be omitted, while still obtaining printing plates with sufficient lithographic properties.
The pre-heat unit, whether or not incorporated in a single processing unit, could then be eliminated. In cases where the pre-heat unit is combined with a development unit in a processing unit, a further simplification of the processing unit would then become possible resulting in a further reduction of the floor space, also referred to as foot print, of the processing unit.
The absence of a pre-heat unit would reduce the cost associated with the processing of photopolymer plate precursors. No pre-heat section has to be put in place, whether combined or not with a development unit in a processing unit, and moreover, less energy will be consumed while processing.
In addition, a pre-heat may result in an inconsistent quality of the obtained printing plates due to inhomogeneous heating of the precursor, or due to fluctuations of the pre-heat temperature as function of the life time of the heating means used.
Another disadvantage of a pre-heat unit when included in a processing unit without a pre-wash unit may be that the temperature of the developing solution increases, resulting in inconsistent and even inferior lithographic properties of the obtained printing plates. Such increase may be minimized by incorporating cooling means in the processing unit as disclosed in the unpublished EP-A 08 102 922.5 (filed 2008-03-26). However, when a pre-heat unit is no longer necessary, the temperature increase of the developing solution will be much less, even in the absence of a cooling means, which will result in more consistent lithographic properties of the obtained printing plates.